DerekL1963

I can't find the exact source, but an American nuclear missile used SRB all the way to space, and had to be stopped at a precise time for their trajectory to take them to the target. To solve the problem, engineers used "venting ports" at the base of the missile, which covers would be blown to redirect thrust to the "sides" of the booster, nullifying forward thrust at that moment. In fact, it worked so well that they were able to snuff the SRB in space like a candle... I'd be very happy if someone knew which missile it was, my google fu is weak this morning. Some missiles use ports in the base dome, other missiles use ports in the forward dome, other missiles blow off the nozzle, other missiles use a combination of two or more of these. Anyhow, the idea isn't to nullify forward thrust (though that can be, and often is, done), it's to reduce pressure in the case. When the pressure drops below a certain critical value (which varies) the burn cannot sustain itself and the motor 'shuts down'. AIUI (from a reliable source) only some of the burn occurs in the solid phase. Mostly the solid sublimes, the resulting gases burn, and heat from those gases sublimes the solid phase. If you vent the gas phase, the burn rate drops essentially to zero. Missiles known to use this: Polaris A-1, Polaris A-2, Poseidon C-3, SUBROC, ASROC, Minuteman-I, Minuteman-II, Minuteman-III, Peacekeeper. (The Titan's for Dyna-Soar and MOL would have used this method thrust termination too...) If you look at this picture [source] of a Posiedon C-3 you can see oval 'patches' in the skin - those are where stacks leading from the forward dome meet the skin of the missile. When it's time to shut down, the dome and the skin are blown and simultaneously the 'bus' (the part with the 'do not touch' sign on it) separates. The motor stays behind (slowed by the venting gasses) while the bus continues on to start releasing the warheads.

The source for that claim, Space Daily, is... not entirely reliable. And if you've paid any attention to space news over the last decade, you'll have noted that the Russians release bold plans to make bold plans on practically a daily basis.

It's not.

In the interest of full disclosure - that SRB also showed one of the downsides of solid motors. Without a shutdown system (which do exist), they provide a fixed amount of d/v, and that amount varies somewhat from motor to motor even with the same batch. New Horizon's Star 48B third stage burned 'hot' and imparted (IIRC) around 5 m/s more velocity than planned.

This. With my eye-hand coordination, without MJ, I'd simply have to delete KSP and walk away. (And before some idiot pops up with the same nonsense they always do... No, it won't "get better if I practice", it's a physical limitation and that's all you need to know.) And there's no One True Way to play KSP anyhow - if there, there wouldn't be mods. The game would be locked down to force us onto that One True Way. The only people who advocate One True Way are people trying to puff themselves up over others. Some people like being Neil Armstrong... Me, I like being George Low, and Bob Gilruth, and Werner von Braun, and Chris Kraft, and Gene Kranz. When MJ touches down a lander I've designed as part of a mission plan I've conceived, I'm rightfully satisfied and proud of myself - the machinery worked just like *I* planned. If someone thinks I'm wrong for feeling so, or cheating for using MJ, or one of the dozen or more the inane variants of the same stupid claim... they're the ones that are the less for it and their ignorance is not my problem.

They're going to have to slow down *somehow*. And if they are in fact skipping the boostback burn and simply going the full ballistic route, it's their only chance to re-target. Unless they're flying due east, a correction in the plane of the flight path will move the impact point both downrange and crossrange. And now I'm having flashbacks to MK5 guidance school....

So what's the most likely failure mode? Breakup during re-entry, or a missed/hard landing? Part of the the function of the boostback burn is to target the vehicles - skipping it means they have to re-target during the entry burn, which takes more energy and is constrained by aerodynamics. It won't take much of an error during the first stage burn for it to be off by miles... so my guess is the most likely failure mode is a clean miss where the vehicle doesn't even come within a couple of miles of the barge. The next most likely is a near miss or failed landing due to excessive horizontal velocity. Much depends on how well they understand the ballistic performance of the vehicle.

Indeed, but a diameter change would have been far better- they need a diameter change if they want to use CH4 Lox and/or plan on 2nd stage reuse. Sure, the performance gain would be enormous, but the rocket could be shortened (yes, there would also be pad modifications), then re-legnthened over time as needed. It would also be much more of a long-term investment- yes, Ch4 is cryogenic, but nowhere near that of deep cryo LO2 The only people afraid of deep cryogens (based on well established, well tested, well known technology) are the amateurs in the cheap seats (who prefer untested, unknown, unproven technologies). Theoretical handwaving from the cheap seats is entertaining - but SpaceX has a business to run today.

The true rouge squadron are the girls of the 501st Joint Fighter Wing - the Strike Witches. (The link is Wikipedia and SFW, have caution googling - some images are not.)

Yeah, I don't think it really was worth the difficulty. Maximizing performance of the first stage means maximizing the payload while still retaining enough reserve to recover to the landing site. Short term PITA, longer term probably at least a medium win.

You could always ask the Magic 8-Ball.

Bimodial Nuclear engines can also produce electricity for the ION drives. *sigh* Which part of that's a fault in the design did you miss? That electricity comes with a huge penalty - the weight of the fuel, fuel tanks, structure, piping and turbopumps, and nozzle. All dead weight being pushed around by a very low thrust engine that will already be challenged to push more than a minimal payload anyhow. I suspect you'd get a little extra thrust from the nozzle used to expel the added propellant and that's about it. NAICT the ion stream doesn't have enough energy to usefully heat, decompose, or otherwise react or interact with much in the way of mass. It is energetic and moving fast, but there's not very much of it in the local exhaust stream at any one time - a few micrograms at the very most I'd think. Nope.

Multimodal nuclear engines are a pretty good deal, actually. The coolant loops you already need in order to run safely as a nuclear thermal rocket can be repurposed to provide electricity without much additional weight. Which still doesn't correct the basic fault in his design - the NTR has a lot of weight that doesn't contribute to making electricity for the ion engine. Saddling an already performance limited propulsion system with dead weight doesn't strike me as a good idea. It's just not a good idea to take two different propulsion systems that are good at two very different things and try to use them on one vehicle.

More complexity for very little gain... especially since the weight is more than just the reactor, you also have to consider the fuel tanks.

That too.

Not really, it doesn't make much sense to use a (incredibly) low thrust ion engine to boost a (very) heavy nuclear engine - nor does it make much sense to toss the nuke stage away after so little use. You could use the nuke as a tug (but spend a lot of d/v getting it back down to LEO), or just go ahead and use it to go to Mars.

15 was cool but didn't all of the last three Apollo's include rovers? Yes.

I think 15 is an interesting mission too, mostly because for the first time the CMP had serious work to do while the LM was on the surface.

By modern standards, he *is* a backwards bumpkin. And a Roman smith would be completely lost in a modern machine shop - because what a modern machine shop does is nothing like what he did. Even in a modern forge or foundry he'd be lost, as he can't read the instructions, if you ask him to anneal something a 1000 degrees - he doesn't know what annealing is, let alone a degree, etc... etc... (Doesn't mean he can't be taught those things mind you, only that trying to transform the Roman production process into a modern one isn't going to be a simple one - they lack the basic concepts and skills.) You should read up on the Luddites sometime. It's not just the scientists and scholars that resist change.

In some cases, not many, we know the exact location of the mine. In others, only the broad general area at best. And it's a dead certainty that there are yet others we have no idea about even the existence of in modern times. When you study history in detail, it's very frustrating how much we *don't* know and have no way of knowing. That too. Refining iron requires more than ore of a known composition, it requires various chemicals and additives to strip out what we don't want and to add what we do want.

Nobody said you couldn't - only that the task is much larger than simply providing information. You need the tools to make the tools - and this case, that includes considerable chemistry (and the relevant analytical equipment). Why? Because you need to know the properties of your inputs in order to predict the qualities of your outputs, that's the basic foundation of modern practical metallurgy. The modern blacksmith doesn't have the chemistry, but he doesn't need it because he either orders materials with known qualities from a catalog, or selects materials 'in the wild' (such as leaf springs) already known to have certain qualities. Someone else has already taken care of that end of the process for him. You'll need the chemistry (and the related equipment) in Ancient Rome because you have neither a catalog nor materials of known quality available - you have to build the whole infrastructure up from scratch.

Define 'concept behind the material.' You don't need a degree in metallurgy or physics to understand 'blend metals, get qualities from each.' They've done it, they've MADE those materials. It means they know the recipes - not the reason why the recipes work, or why they sometimes don't work. They don't know that adding carbon makes steel, they know "heating in charcoal makes this particular metal harder". The don't know adding [whatever] makes the material more corrosion resistant, they know "adding a capful of sand collected from the beach of La Mad at high tide will reduce rust". Knowing how to make something like steel isn't the same as knowing why what you do causes the changes in the materials properties. The latter is a huge (and largely unrecognized) part of the Industrial Revolution and the modern world - once you know why, you have the ability not only to make material in industrial quantities, but make materials of predictable qualities in industrial quantities.

Yes, they could climb without skipping. The PAD is for a normal entry. How I read the PAD is they've used the data slots for what appears at first glance to be a skip trajectory because lofting is the first step of a skip - but you can stop short (that is not apply enough lift to actually exit the atmosphere) and still move the landing point downrange (just not as far as a full skip). Which is what they did - stopped short of skipping. The comments appended by the journal editors to the interpretation of the PAD describe just that - using P65 (entry lift control) for the loft phase, then skipping P66 (ballistic guidance during the exoatmospheric portion of the skip) since they didn't actually skip, and going straight into P67 (final entry).

That's blatantly untrue. Your average Roman forgemaster would know about brass, bronze, electrum, meteoric iron (though might not group it as such), pewter, sterling silver, wrought iron, pig iron, and steel (though the concept of making structural materials out of the metal of swords and blades might take explaining). Knowing the existence of the material isn't even remotely the same thing as understanding the concept behind the material.

And it's more than the stuff on the factory floor... The goal is to make industrial quantities of steel with predictable qualities. Which means you need lab equipment too, and all the industries needed to produce it. Ultimately, that's the thing that most people miss in these alt-history scenarios, most any technology (and especially anything resembling modern technology) is a spiderweb of interdependencies. Most of them invisible to the casual observer.